Geologic formations may be used for many purposes such as hydrocarbon production, geothermal production or carbon dioxide sequestration. In general, boreholes are drilled into the earth to gain access to the formations to perform measurements related to the intended purpose of the formation. Measurements performed in boreholes at various depths are generally referred to as well logging.
In one embodiment of well logging referred to as logging-while-drilling or measurement-while-drilling, a logging instrument or tool is disposed at a bottomhole assembly (BHA) at the distal end of a drill string. As the drill string rotates to drill the borehole, the logging tool performs the measurements. The measurements are generally logged with a depth at which they were performed. Together, the measurements and associated depths create a well log. The well log may be presented graphically or as an image of changes in measured properties.
One type of downhole measurement is of electrical properties of the earth formation. The electrical properties may provide information about the geologic materials that make up a formation, such as their likely oil, gas and water content.
The electrical properties of the earth formation may be measured by an induction well logging technique. In induction well logging, a transmitter transmits electromagnetic energy through a transmitter antenna into a formation. The changing magnetic fields associated with the energy induce eddy currents in the formation. The eddy currents in turn emit electromagnetic energy that is received by a receiver via a receiver antenna. Characteristics of the received electromagnetic energy are then related to the electrical properties of the formation. For example, a formation with greater conductive properties will have higher magnitude eddy currents generating stronger signals. Unfortunately, with this type of well logging, the depth of measurements into the formation may be limited by the range at which the electromagnetic energy can penetrate the formation rock. Hence, it would be appreciated in the drilling industry if induction logging tools could be improved to perform measurements deeper and more accurately in geologic formations.
Challenges are presented when performing induction logging in a while-drilling application because an induction logging tool performing the measurements rotates with the drill string as the borehole is being drilled. With each revolution of the drill string, the induction logging tool is conveyed deeper into the borehole. If measurements are performed slowly, then the measurements may miss parts of the formation or measurements may not be able to be correlated with an azimuthal direction from the borehole. Hence, it would be appreciated by the drilling industry if induction logging tools could be improved to perform measurements more quickly.